In a disc drive system, an actuator is used to position a read/write head over data storage areas, the actuator being driven by a voice coil motor (VCM). A latch is sometimes used for holding the actuator at a latched position when, for example, the disc drive is not in operation.
To unlatch the actuator from the latch, the conventional method is to apply steps of current progressively to the VCM in the direction opposite to the latching force, until the actuator breaks away from the latch. Once it is unlatched, the actuator would tend to move to the other extremity of its range of travel, and may even crash into an outer crash stop. This is undesirable as it introduces shocks and risks damage to the read/write head and the data storage areas.
One method of restraining the motion of the actuator is described in the U.S. Pat. No. 5,600,219 to Kevin A. Gomez granted Feb. 4, 1997 for "Sensorless Closed-Loop Actuator Unlatch". Through monitoring the slope of the back EMF (electromagnetic force) of the VCM, the actuator unlatch controller is able to determine when the actuator has broken free of the latch, at which point the actuator latch controller would cause the current signal to the voice coil motor to be cut off.
As the requirement for shock-resistance is pushed higher, it has resulted in an increase in the latching force with which the actuator is held at the latched position. Accordingly, the current to the VCM has to increase to a higher and higher level in order to unlatch the actuator. However, the problem of the actuator accelerating beyond control upon release is aggravated as the current to the VCM is increased. Cutting off the current to the VCM once the actuator is unlatched does not provide an adequate solution to the problem. It follows that there is a need for an improved method of unlatching the actuator.
The present invention not only provides an improved method and apparatus for unlatching the actuator suitable for use with disc drive systems with large latching forces, it also provides a faster method of unlatching the actuator. It will be evident from the following description that the present invention offers other advantages over the prior art.